We have obtained high-precision interferometric measurements
of the A0 V standard star Vega with the Center for High
Angular Resolution Astronomy (CHARA) Array and the Fiber
Linked Unit for Optical Recombination (FLUOR) beam combiner
in the K' band at projected baselines between 103 m and 273
m. The measured squared visibility amplitudes beyond the
first lobe are significantly weaker than expected for a
slowly rotating star and provide strong evidence for the
model of Vega as a rapidly rotating star viewed very nearly
pole on. We have constructed a Roche-von Zeipel
gravity-darkened model atmosphere which is in generally good
agreement with both our interferometric data and archival
spectrophotometry. Our model indicates Vega is rotating at
~92% of its angular break-up rate with an equatorial
velocity of ~275 km s-1. We find a polar effective
temperature of ~10150 K and a pole-to-equator effective
temperature difference of ~2500 K, much larger than the
~300 K derived by Gulliver, Hill, and Adelman. Our
model suggests that Vega's cool equatorial atmosphere may
have significant convective flux and predicts a
significantly cooler spectral energy distribution for Vega
as seen by its surrounding debris disk.

This work was performed in part under contract with the Jet
Propulsion Laboratory (JPL) funded by NASA through the
Michelson Fellowship Program. JPL is managed for NASA by the
California Institute of Technology. The CHARA Array is
operated by the Center for High Angular Resolution Astronomy
with support from Georgia State University and the National
Science Foundation, the Keck Foundation and the Packard
Foundation.